Solar Cells in Action – Teaching Guide
If only I were driving the solar cell buggy – nobody would catch me...
What this guide offers you.
Here are teaching and learning ideas for you to use with S1 and S2 pupils. The sequence will occupy about three lessons and allows learners to explore solar cells through their use in powering a solar buggy.
Contents: 1. 2. 3. 4. 5. 6. Introduction to the buggy and how you might use it Suggested scheme of work including timings Materials for your pupils Background physics Why is this work of value? The Optoelectronics College
2 4 6 7 8 8
1. Introduction to the buggy and how you might use it.
The Solar Buggy Kit The buggy comes in kit form and is available from Middlesex University Teaching Resources (www.mutr.co.uk). The retail price of the „solar powered buggy kit‟ was about £15 in 2008. You may need to assemble the buggy from the kit (not difficult if you follow the directions in the “Solar Cells in Action Teacher and Technician Guide”). A 10 F capacitor is used on the buggy which enables energy to be stored over time and then released once the capacitor is discharged across the motor. We suggest that this is introduced to learners as a „capacitor‟ and that its purpose is to act as an „energy store‟. We appreciate that capacitors will not have been met before but feel that giving the correct identity doesn‟t present an intellectual hurdle. (A series diode prevents the capacitor from discharging across the solar cell.)
Storing energy Unclip the crocodile clip and place the buggy in direct sunlight or under a bright light source. We found that a few minutes charging will allow the buggy to move slowly. Longer charging times under brighter sources will result in greater travel distances.
Releasing the stored energy Use the crocodile clip to complete the discharging circuit and thereby release the stored energy. A Powerpoint slide set “Introduction to the solar buggy” shows the buggy in storing and moving modes. This can be used by teachers or by pupils.
Additional explorations The kit also includes sets of coloured filters, thin cards, surface area mask and alternative light sources. You may wish to supplement these resources but the kit can be used as it stands. The solar cell reaches peak sensitivity in infrared wavelengths. The „hot‟ tungsten filament lamps give out sufficient infrared radiation to energise the buggy, even with card masks apparently placing the solar cell in shadow. The energy saving fluorescent lamp does not emit much infrared and is less efficient in energising the buggy. Infrared radiation passes through the coloured filters and through paper and thin card. Pupils investigating with cards and filters are therefore likely to discover that the buggy is energised even though a card blocks the visible light. This is an opportunity to introduce the idea of „invisible radiations‟ to learners. We include a metallic mask in the kit which will absorb the infrared radiation. Investigation of the effect of colour of light on the solar buggy should be done using the colour filters and the fluorescent lamp. If the colour filters are used with a tungsten lamp Page 2
the large amount of infrared emitted by the lamp will swamp the effects of the absorption of the different colours by the colour filters.
The World Solar Challenge “The Challenge: Design and build a car capable of crossing the vast and imposing continent of Australia using only sunlight as fuel and to prove it, in the spirit of friendly competition against others with the same goal.” http://www.wsc.org.au The race attracts teams from around the world, most of which are fielded by universities and corporations, although a few high schools have also entered! The inaugural event took place in 1987 with nine races having been held to date. The event is held on public roads between Darwin and Adelaide, a distance of approximately 3000km, with last year‟s winner driving at an average speed of 90km/h.
2. Suggested scheme of work including timings
Our suggestion is that this work occupies at least three teaching sessions of about 60 minutes duration. The class kit allows for five parallel groups each with a buggy and investigation kit. Teachers will want to find their own route through the materials but we suggest the following sequence as a starting point. Lesson 1: setting the scene 1. You may want to show the World Solar Challenge video clip - Running on Sunshine (4 mins long) as a scene setting activity. Issues to be raised include: Energy transfer from Sun to car How challenges flag up new technologies Pollution free modes of transport „Free‟ and unlimited energy from the Sun Streamline features of such a vehicle 2. Introduction to the buggy. There is a Powerpoint slide sequence “Introduction to the Solar Buggy” illustrating the use of the solar buggy. 3. Solar cell investigation. Learners can find out how to operate the buggy (charging, use of the switch etc) and explore how far a buggy will travel for a particular charge time, say 5 minutes. This will be helpful for Lesson 2. Lesson 2: investigations (an alternative version of lesson 2 using additional apparatus is describes in Appendix 1) The five buggy groups, each with a buggy kit, can investigate one of a number of variables which may contribute to changing the motion of the buggy (by varying the energy stored). The variables which the kit offers are: i. Area of cell exposed ii. Angle of solar cell / illumination iii. Simulated cloud conditions iv. Type of light source v. Colour of light ( using filters) vi. Lamp to solar cell distance vii. Time of exposure With each investigation the distance travelled by the buggy is the dependent variable. It is expected that the different groups of researchers, having investigated one of the variables, will offer a summary of their work to the whole class. As a result the whole class can gain a shared understanding of the behaviour of the solar buggy with the different variables.
Lesson 3: the buggy challenge
This is the culmination of the solar buggy work. With data from the lesson 2 investigations the groups are now able to face a challenge. We suggest that the challenge be along the lines of “Prepare your buggy to move a particular distance and stop at a predetermined place.” The teacher should select a distance for the pupils to aim at to fit in with space available in the laboratory and expected charging and driving times in relation to the period time available. Our investigations suggest that a distance of less than 3 m is manageable both in terms of time and laboratory space. A variation to this challenge would be to state particular conditions such as: .....to stop at 2 m but „you must use only half the solar cell area‟ or „ you must use a coloured light source‟.
In order to minimise the effects of differences between the buggies in the kit it is recommended that each buggy be numbered and each group always work with the same buggy. In practice, the friction of the motor, gears etc varies significantly and if pupils are to predict performance for the challenge consistent use of the same buggy is an advantage.
In summary, the teaching sequence we suggest is as follows:
1. World Solar Challenge and buggy introductions
2. Solar cell investigations Area of solar cell exposed Angle of light source Simulated cloud cover Type of light source Colour of light Distance from lamp Time of exposure
3. The buggy challenge and plenary reflections
3. Materials for your pupils
To complement the practical buggy kit a number of resources for supporting learners have been developed. These resources, along with additional ones as they become available, will be available at www.opto.org.uk. Solar Cells in Action activity kit Solar Cells in Action Teacher and Technician Guide Running on Sunshine - solar car race (video) Introduction to the Solar Buggy (Powerpoint) Investigation Concept Cartoon (Powerpoint) Two sets of starter discussion slides (one cartoon style, one bullet points in Powerpoint) How Solar Cells Work (Powerpoint)
The concept cartoon and cartoon style starter slides are based on an original idea for concept cartoons from Millgate House Education, www.millgatehouse.co.uk
4. Background physics
Solar cell The solar cell works when light falls on its surface. The material at the cell surface is silicon and is a type of „semiconductor‟. On striking the semiconductor surface the energy of each particle (photon) of light is transferred to an electron in the semiconductor. The result is a build-up of electrons within the semiconductor. The build up of electrons provides an electric force which redistributes charges in the capacitor. The solar cell therefore „charges up‟ the capacitor rather like a rechargeable battery. A Powerpoint presentation “How Solar Cells Work” gives some background information on the operation of solar cells. This Powerpoint includes some animations which give a visual model of the behaviour of photons and electrons in the semiconductor material of the solar cell. Energy We suggest that the energy stories told during this activity refer to energy as being stored and/or transferred. The exact nature of the „energy‟ concept is unclear to many learners and teachers. It has been the subject of much discussion in the scientific community. Transfer and store are terms in current use and avoid the thorny issue of „energy forms‟. Capacitors A capacitor is a device which, when „charged‟, holds a redistribution of electrons on two pieces of conductor separated by an insulator. The solar cell creates an electric force which drives the redistribution process. For our purposes learners need know nothing about the working of a capacitor other than that it acts as an energy store for the solar cell. (The electric force mentioned in this explanation of the working of a capacitor might be referred to as an electromotive force, potential difference or voltage by physics teachers.)
5. Why is this work of value?
There are several learning outcomes associated with this work. Learners will be able to: Value working in a team and learn about their contribution to and relationship within the team effort. Develop and refine their problem solving skills Practice their skills of measurement – especially of time and distance. Describe the buggy and lamp system as an energy transfer process using the terms „energy transfer‟ and „energy store‟. Appreciate that infrared radiation is invisible Give examples of where solar cells are used to transfer energy Enhance their appreciation of the role of clean/alternative energy sources
The work relates directly to a draft outcome within the Curriculum for Excellence: CfE outcome I have participated in constructing a model to harness a renewable source of energy, and can investigate how to optimise the output. SCN 310G By drawing conclusions from my data and evaluating my design I can consider the commercial potential of this source.
6. The Optoelectronics College
This work was funded by the Rank Prize Fund through the inception of The Optoelectronics College, founded by Prof. Ian Shanks and Prof. Wilson Sibbett in 2007. The aim was to make teaching and learning of science more up-to-date and engaging and potentially to encourage greater take up of science subjects post-16. The strategic plan includes: Linking teachers and learners with a community of researchers Making use of the emerging new technologies found in the real world Engaging with teachers via a programme of CPD Partnership with equipment designers to support new apparatus Targeting S1 / S2 learners
This initiative is supported by the Optoelectronics College website: http://www.opto.org.uk (currently under development)
An alternative for lesson 2: Investigations In some situations teachers might find it appropriate to investigate the behaviour of solar cells in isolation from the buggy. This has certain advantages: this may provide an opportunity for more explicit teaching of investigative skills it may allow a clearer identification of variables the activity can be done in pairs, rather than larger groups the activity uses less space
The same input variables can be investigated in isolation as can be investigated on the buggy. However, in this case, the dependent variable is the output voltage of the solar cell. While this is different from the dependent variable measured on the buggy (distance travelled), there is sufficient correlation between output voltage and energy stored in the capacitor as to make this a valid measurement. However, this approach requires the purchase or availability of additional solar cells and voltmeters.
Equipment Each group requires: a solar cell (those used on the solar buggy are available from SEP/MUTR for a cost of less than £3 each in 2008) a voltmeter – an analogue voltmeter or a multimeter can be used, but it is probably easiest to use an “easy-read” digital voltmeter
The lamps, coloured gel, paper and metal masks from the Solar Cells in Action kit may be used to allow the same range of variables to be measured as in the original version of Lesson 2 using the distance travelled by the Solar Buggy as the dependent variable.
Procedure Each group should investigate one variable as described on page 4. This investigation provides a good opportunity to discuss how many readings should be taken in order to draw a valid conclusion. Consideration should also be given to how data will be recorded, and to which type of graph can be used to present data most clearly, thereby allowing discussion of continuous and discontinuous variables. For some groups (investigating distance, angle and area) it will be more appropriate to draw line graphs from results. Other groups (investigating type of light source, colour of light) could plot bar graphs. Those investigating cloud cover with different grades of paper could use either depending on whether quantitative information is given about the types of paper.
Groups should be encouraged to draw a conclusion from their results and groups could spend additional time producing more formal reports on their investigation. Findings can be shared with other groups in a number of ways: A member of each group presents the results and conclusion to the whole class “Expert gallery” (a co-operative learning technique) “Envoys” could share information between groups
Each group can then discuss the findings of all groups before deciding on their own strategy for the buggy challenge.